Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene
Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene
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DOI：
10.1063/1.481478
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发表时间：
2000-05-15
影响因子：
4.4
通讯作者：
Trushin, SA
中科院分区：
文献类型：
--
作者：
Fuss, W;Schmid, WE;Trushin, SA
研究了1，3-环己二烯在气相中的配位光化学开环，采用267 nm的低强度泵浦和800 nm的高强度光电离探测，并对离子产率进行了质量选择检测。我们发现了五个不同的时间常数，它们可以被分配给沿着势能面的连续部分的旅行时间。分子首先在光谱态1B中沿着Franck-Condon活动坐标加速，然后改变方向，然后转换到暗态2A。所有的常数，包括离开2A表面的常数，都在100fS以下。这些时间比适当的振动周期要短。这样的最大速度证明，该路径是通过真实的交叉(圆锥形交叉)连续地从表面通向表面，并且分子被直接加速进入2A/1A漏斗的出口。在基态，它以紧凑的波包形式到达，表明某种程度的相干。这种实验方法在研究动力学方面有很大的潜力，因为可以检测到这个过程的许多连续阶段。这是因为碎裂模式取决于势能面上的位置，因此监测几个不同的离子可以得出通过这些位置的布居流动的结论。在使用的强度下的电离通常被认为是辐射电场的影响。但在我们的例子中，它被中性分子中的共振增强，特别是在单阳离子中，它对分子的长度(产物己三烯的不同构象)不敏感。离子共振解释了为什么己三烯的碎裂模式比环己二烯丰富得多。库仑爆炸是从双正离子的激发态观察到的。并对其机理进行了探讨。(C)2000年美国物理研究所。[S0021-9606(00)00819-9]。
The concerted photochemical ring opening of 1,3-cyclohexadiene was investigated in the gas phase by low-intensity pumping at 267 nm and subsequent probing by high-intensity photoionization at 800 nm and mass-selective detection of the ion yields. We found five different time constants which can be assigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B along Franck-Condon active coordinates, then alters direction before changing over to the dark state 2A. All constants including that for leaving the 2A surface are below 100 fs. These times are shorter than appropriate vibrational periods. Such a maximum speed is evidence that the pathway is continuous leading from surface to surface via real crossings (conical intersections) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicating a certain degree of coherence. The experimental method promises a high potential for investigating dynamics, since many consecutive phases of the process can be detected. This is because the fragmentation pattern depends on the location on the potential energy surface, so that monitoring several different ions permits to conclude on the population flow through these locations. Ionization at the intensities used is normally considered to be an effect of the electric field of the radiation. But in our case it is enhanced by resonances in the neutral molecule and in particular in the singly positive ion, and it is not sensitive for the length of the molecule (different conformers of the product hexatriene). The ionic resonances explain why hexatriene has a much richer fragmentation pattern than cyclohexadiene. Coulomb explosion is observed from an excited state of a doubly positive ion. Its mechanism is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)00819-9].